It is known that information may be transmitted over an optical link having a light transmitter, a light conducting element and a light detector. Typically, the radiation output of the light transmitter is modulated in accordance with an input signal and the modulated light is transmitted over a light conducting element, such as an optical fiber, to a detector, for example a photodiode, that converts the modulated light to corresponding electrical signals that are proportional to the input signal. The input signal may be generated by a transducer that is employed to measure some physical quantity.
In a measuring device, the signal of the transducer may be applied to modulate the radiation of a light transmitter, such as a light emitting diode on a transmission side of the device. The radiation of the light emitting diode is then transmitted over an optical fiber to a receiving side of the device, wherein the modulated light signal is converted to a corresponding electrical output signal by a photodiode.
The optical transmission of data between a transmitter side and a receiver side of a measuring device is advantageous in that the transmitter and receiver are not electrically connected and, therefore, may be operated at different potential levels. In addition, a data transmission over an optical link cannot be blocked by electromagnetic radiation, while an electrically conducting signal line is sensitive to such radiation. Also, since an optical link does not transmit electrical energy, sparks or short circuits cannot occur and, therefore, an optical link will be much safer than an electrical signal line when used in an environment having combustible or explosive material.
It is highly desirable to reduce the power consumption on the transmitter side of a measuring device as much as possible, since it is often difficult and expensive to supply a great deal of power to the transmitter, especially if the transmitter is located at a remote measuring location where electrical power is not readily available. The requirement of low transmitter power makes it particularly difficult to transmit digital measuring data over an optical link, since a digital transmission normally requires high power digital-to-analog converters and high power light modulators on the transmitter side.
Conventional analog transmissions also have a disadvantage in that the accuracy of a transmission is dependent upon signal attenuation in the optical link, and the attenuation is known to vary for many reasons. For example, a change in the radius of curvature of a light conductor will attenuate a transmitted signal and changes in the amplification of light emitters and light detectors, due to temperature conditions and aging, will also attenuate the signal. Another disadvantage of an analog transmission scheme is that the dynamic range of the analog signal will tend to be small if available power on the transmitter side is low.
Accordingly, it is a primary object of the invention to provide a measuring device having a high accuracy, a large dynamic range and a large band width, and utilizing a low power transmitter that is connected to a receiver by an optical link.
Another object of the invention is to provide such a measuring device including means for automatically compensating for variations in signal attenuation along the optical link.
These and other objects of this invention will become apparent from a review of the detailed specification which follows and a consideration of the accompanying drawings.